This invention relates to in situ processing of oil shale.
The presence of large deposits of oil shale in the Rocky Mountain regions of the United States have given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. A number of methods have been proposed for processing the oil shale which involve either first mining the kerogen-bearing shale and processing the shale on the surface, or processing the shale in situ. The latter approach is preferable from the standpoint of environmental impact, since the treated shale remains in place, reducing the chance of surface contamination and the requirement for disposal of solid wastes.
A typical method (modified in situ recovery or MIS) of in situ shale oil recovery involves the mining of approximately 20-35 percent of the underground chamber or retort, and then explosively rubblizing the remaining shale in the retort to fill up the mined out area. The rubblized shale is then retorted by igniting a combustible gas and air (or O.sub.2) at the top end of the retort so as to initiate a combustion front at the top of the retort. Once combustion has been initiated, only air (or O.sub.2) is fed into the retort, combustion gas being generated by decomposition of kerogen. The combustion front passes down through the retort, retorting the shale therein so as to produce shale oil. Shale oil flows down the retort chamber by gravity, and is collected in a sump at the bottom of the retort for subsequent transport to the surface. The MIS process as described above is described in U.S. Pat. No. 4,167,291 of Ridley, whose disclosure is herein incorporated by reference. There are three major problems in prior in situ techniques which lead to inefficient recovery of shale oil.
The first problem associated with such a technique is the inability in prior methods of achieving a flat combustion front.
It has been found that the best yield of shale oil is obtained when the above mentioned combustion front is substantially flat, or perpendicular to its direction of advancement. If the front is skewed or warped, part of the front will reach the bottom of the retort ahead of the remainder of the front. The air will then have a tendency to flow out the retort through the area of the retort bottom where the front has broken through. Therefore, movement of the front will cease due to lack of supply of air, leaving a substantial portion of the rubblized shale unretorted.
The primary cause of a skewed or warped combustion front is non-uniform gas flow properties of the rubble bed due to spatial variation in void fractions. As used herein the term "void fraction" refers to the ratio of the volume of the voids or spaces between particles in the fragmented mass to the total volume of the fragmented permeable mass of particles in an in situ oil shale retort.
Inhomogeneities in void fractions stem from a tendency for oil shale fragments formed and displaced by the explosive blast to translate and rotate differently in one portion of the retort than in other portions. For example, fragments formed near the lateral boundaries of the retort tend to rotate more than fragments formed in the interior of the retort. The result of this is that the void fraction near the retort boundary is greater than in the retort interior. Permeability follows the same trend so that a combustion front which starts out being nominally flat at the top of the retort becomes umbrella-shaped as it moves downward and eventually breaks through the retort bottom first around the lateral boundaries. The result is that a significant portion of the retort (perhaps as much as 30 to 40%) may not be swept by the combustion front before breakthrough occurs and hence will not be retorted under optimal conditions. This particular difficulty in in situ oil shale retorting is generally regarded as the single most important cause for oil recovery efficiencies to be depressed below a level which would be theoretically possible.
The second major problem encountered in prior in situ techniques involves the undesirable burning of the larger shale particles. On a local scale (i.e., a region large enough to encompass one or two of the largest fragments and as many of the smaller fragments as may be in that region), the combustion front will move through the small fragments before the kerogen in the large fragments has been fully converted into shale oil. The result is that product from the large fragments is formed behind the combustion front and thus is apt not to be recovered, but rather burned.
The third major problem is the non-homogenity of the shale itself (i.e., layers or inclusions of clay and variations of kerogen content of the shale). When the combustion front reaches a layer of clay or shale having low kerogen content, lean shale, the front may slow or even cease.